Thallium-bromide (TlBr) is a compound wide-bandgap semiconductor considered to be a promising room temperature X- and gamma-ray detector material due to its excellent spectroscopic performance, with resolutions of <1.2% having been demonstrated. Compared to the more common CZT (CdZnTe) room temperature detectors, TlBr is known to have about three times the gamma sensitivity, suggesting only one-third the volume of CZT would be needed to achieve a comparable response to gamma rays.
However, the use of TlBr as a room temperature radiation detector is limited by polarization of the TlBr crystal under applied bias causing a steady reduction in the internal field at room temperature and instability in dark current. The polarization phenomenon is attributed to the fact that TlBr is a mixed ionic-electronic conductor (“MIEC”). At room temperature, a large concentration of Schottky pairs of Tl and Br vacancies (VTl−, VBr+) can easily form due to their low formation energy. Under an applied bias and at room temperature, ionic current can be significant and dominated by the migration of Br vacancies (VBr+), which have an extremely low energy of migration, rendering it highly mobile within the lattice. FIG. 1 illustrates Br vacancy migration toward the cathode in a TlBr crystal 10 under applied bias (anode not shown), and having a surface damage layer 11 where there is lower mobility. The resulting imbalance in the distribution of charged vacancies results in the build-up of an internal electric field that opposes the applied bias and thereby gradually decreases current and carrier collection efficiency (as shown by the graph charting current density in time in FIG. 1) and degrades spectroscopic performance.
Several approaches to reducing polarization and increasing stability are known. One approach is to cool the detectors down to about −15° C. or lower, which serves to decrease both the vacancy concentration and their mobility. Detectors operated at sufficiently lower temperature show no signs of degradation, even under long-term applied bias. Another approach is to apply Tl metal contacts to the crystals, and reverse the voltage polarity roughly every 24 hours in order to maintain stable operation. While both of these approaches serve to increase the operational lifetime of TlBr detectors, the first approach can be power intensive for portable devices due to the cooling requirement, and the second requires the use of Tl metal, which is highly toxic and can be readily absorbed through the skin. Moreover, switching of the voltage polarity adds to the complexity of the circuitry thus adding cost, and may not be compatible with single carrier charge sensing techniques.
Surface quality of TlBr is also known to be a contributing factor of TlBr performance as a radiation detector. Because TlBr is a soft material, dislocations, non-equilibrium Tl and Br vacancies, and other structural defects are generated during crystal growth and mechanical processing (e.g. cutting and surface polishing) which can affect the electrical resistance, carrier diffusion and trapping, and deteriorate the quality of TlBr detectors. In a common process of TlBr preparation, a bromine methanol solution is used to chemically etch a mechanically polished TlBr surface to remove surface defects. And as described in the article “Surface Processing of TlBr for Improved Gamma Spectroscopy” by Voss et al (2010 IEEE), incorporated by reference herein, H2O2 etchant has also been used to remove surface damage. However, the use of both bromine-based etchants and H2O2 etchants can result in large, long lived current transients due to a buildup of the far more mobile Br vacancies at the cathode (negatively biased electrode), which lowers the barrier to electronic injection and degrades performance, as illustrated in FIG. 2 showing an example TlBr crystal 12 under applied bias and having an etch processed surface 13. FIG. 2 also shows a graph charting current density in time and showing the domination of electronic injection due to build up of extrinsic vacancies. While after a period of time, the current eventually returns to its original value (i.e. “field annealing”) and does not increase again and is relatively stable, during this period of high current the TlBr detector is inoperable due to the large noise floor. Additionally, the detectors eventually begin to degrade even when this transient has passed, thus limiting the operational lifetime
Thus, an alternative method to control the polarization of these MIEC-based detectors is desired which address and controls the polarization phenomena and substantially eliminates large current transients to enable long term stability and operation.